Lithium ion secondary batteries, which are small in volume and large in mass capacity density and provide a high voltage, have been widely employed as power supply sources for small devices. Lithium ion secondary batteries are, for example, used as power sources for mobile devices such as mobile phones and notebook computers. Recently, in addition to the application to the small mobile devices, since environmental concern and energy conservation trend are increased, lithium ion secondary batteries have been applied to large secondary batteries required to have a large capacity and a long life to be used in the fields of electric vehicles (EV), power storage and the like.
In the lithium ion secondary batteries currently on the market, a positive electrode active material based on LiMO2 (M represents at least one of Co, Ni and Mn) of a laminate structure or LiMn2O4 of a spinel structure is generally used. A lithium ion secondary battery having such a positive electrode active material uses a charge/discharge region of principally 4.3 V or less (hereinafter, a positive electrode containing a positive electrode active material having an operating potential of 4.3 V or less relative to a lithium metal will be sometimes referred to as a “4 V-level positive electrode”). As a negative-electrode active material, a carbon material such as graphite is used.
In the meantime, a material obtained by substituting Mn of LiMn2O4 partially with Ni and the like is known to exhibit a charge/discharge region as high as 4.5 to 4.8 V relative to a lithium metal. More specifically, a spinel compound, such as LiNi0.5Mn1.5O4, does not use conventional redox between Mn3+ and Mn4+ but uses redox of Ni2+ and Ni4+, since Mn is present in the state of Mn4+. Because of this, the spinel compound exhibits an operating voltage as high as 4.5 V or more relative to a lithium metal. Such a material, which is called as a 5 V-level active material and can improve energy density by applying a high voltage, is expected as a promising positive electrode material. Hereinafter, a positive electrode comprising the 5 V-level active material will be sometimes referred to as a “5 V-level positive electrode”.
However, if the positive electrode potential increases, reduction in battery capacity and gas generation become significant due to oxidative decomposition of the electrolyte solution. Because of this, there is a problem in the application of a generally-used electrolyte mainly containing a carbonate-based solvent. In the circumstances, in order to suppress decomposition of the electrolyte solution at a high potential, investigation as to whether a solvent having high oxidation resistance is applied to an electrolyte solution of a lithium ion secondary battery comprising a 5 V-level positive electrode has been made. For example, a fluorine solvent such as fluorinated ether and fluorinated phosphoric acid ester and a sulfone solvent such as sulfolane have been reported as highly oxidation-resistant solvents. In particular, it is believed that a fluorine solvent is excellent in oxidation resistance.
However, since the above solvents that can be used in a battery using a 5 V-level positive electrode (hereinafter referred to also as a 5 V-level battery) cause decrease in ion conductivity and increase in viscosity, as compared to a carbonate solvent, they have a problem of deterioration of rate characteristics (output characteristics) of the battery. Even if an oxidation-resistant solvent is used, the following problem still remains in a 5 V-level battery: life characteristics such as cycle characteristic and storage characteristics are lower than those of the battery using a 4 V-level positive electrode (hereinafter referred to as a 4 V-level battery). This problem prevents practical use of the 5 V-level battery.
When sufficient performance cannot be obtained by a single type of battery alone, a method is considered in which a second and a third battery different in type from this are prepared and these plural types of batteries are controlled in combination to improve the characteristics as a battery system (Patent Literature 1).